Forging
Forging shapes hot or cold metal by compressive force. Learn open- and closed-die forging, hot vs cold working, grain flow, true strain and flow stress, forging-force estimation and design rules.
A blacksmith hammering red-hot iron is forging. Industrial presses and hammers do the same at huge scale to make crankshafts, connecting rods, gears, hand tools and aircraft landing gear.
Forged parts are stronger and tougher than cast or machined parts because the metal's grain flow follows the part shape and internal voids are closed.
An analogy
Kneading and pressing dough into a mould: the material flows but isn't removed, and its internal structure lines up with the shape.
Open-die and closed-die
Open-die: flat or simple dies, the operator moves the workpiece — large shafts, rings, blocks. Closed-die (impression-die): the metal fills a shaped cavity, excess escapes as flash which is trimmed off — connecting rods, spanners, gears.
Hot, warm and cold forging
Hot forging (above recrystallisation, steel ≈ 1100–1250 °C) needs low force and allows big shape changes but gives scale and looser tolerances. Cold forging (room temperature) gives precise, strain-hardened parts (bolts, gears) but needs high forces and ductile materials.
Grain flow and properties
Forging aligns grain flow with the part outline, improving fatigue and impact resistance — why safety-critical crankshafts, wheel hubs and aircraft fittings are forged.
Force, friction and simulation
Friction at the die increases force and causes barrelling. Force estimates use flow stress (temperature- and strain-rate-dependent in hot forging) times area with a friction multiplier. FEM (DEFORM, QForm, Simufact) predicts die filling, laps, and die stresses to optimise preform stages.
Formulas
- True strain (upsetting):
ε = ln(h₀/h) - Volume constancy:
A₀h₀ = A h - Open-die forging force:
F = Y_f · A · (1 + μd/(3h)) - Flow stress (cold):
Y_f = K εⁿ
Worked example: Upsetting force
Given: Billet Ø50 × 100 mm upset to 50 mm height; Y_f = 300 MPa (hot), μ = 0.2
Find: Final force and true strain.
- A = 1963 × 2 = 3927 mm², d = 70.7 mm
- Friction factor = 1 + 0.2 × 70.7/(3 × 50) = 1.094
- F = 300 × 3927 × 1.094 = 1.29 MN
- ε = ln 2 = 0.69
Answer: ≈ 1.29 MN (≈ 130 tonnes), ε = 0.69
Worked example: Volume constancy
Given: Bar Ø30 × 200 mm forged to Ø20
Find: New length.
- L = 200 × (30/20)² = 450 mm
Answer: 450 mm
Common mistakes
- Wrong: Sharp corners and deep narrow ribs in closed-die parts. — Right: Use generous radii, draft (3–7°) and adequate rib widths so metal fills without laps.
- Wrong: Ignoring flash in material estimates. — Right: Allow 10–30 % extra for flash and scale.
Applications
- Automotive: Crankshafts, connecting rods, steering knuckles, gears.
- Aerospace: Landing gear, turbine discs, titanium structural fittings.
- Tools & fasteners: Spanners, hammers, bolts (cold heading).
Key takeaways
- Forging reshapes without removing material; volume is constant.
- Hot = low force, big deformation; cold = precision + strain hardening.
- F ≈ Y_f A (1 + μd/3h).
- Grain flow gives superior toughness and fatigue life.
Going further
- Isothermal forging: Dies heated to workpiece temperature allow near-net titanium and nickel parts with low force.
- Precision (net-shape) forging: Gears with forged teeth needing little or no machining.
- Microstructure control: Controlled cooling after forging (micro-alloyed steels) replaces separate heat treatment.